The breakage of tempered glass is a general issue that has been misapprehended among users, installers, and glass producers. Majority of people understand that tempered glass could not be broken, and accordingly believe that tempered glass is one-hundred percent secure. In fact, tempered glass could break due to various causes, e.g., if a broken tempered glass has no frame or attachment at the side of glass (such as silicone), the glass will definitely disperse out. This could cause injuries or even require sewing up a wound in case of being close to the glass while breaking, yet there is no danger up to the stage of severe injuries or death.

Each type of glass has dissimilar advantages and disadvantages; selecting a suitable type of glass requires a consideration in term of desired purposes. Areas that are often being hit, tempered glass is necessary to be applied. If desire to increase difficulty in smashing glass, laminated glass is the most appropriate one. In case of preventing heat transfer and noise, insulated glass is required.

Intertech Specialty Glass Co., Ltd, a producer of tempered glass, laminated glass and insulated glass for construction, would like to provide fundamental information about tempered glass via Q&A as follows.

Question: How long does tempered glass last?

Answer: It lasts forever as long as it has not been smashed (such as during an installment process) or included with nickel sulfide.

Question: Can environmental factors lead to glass breakage in tempered glass?

Answer: If temperature does not exceed 290 ºC and there is no rapidly change of temperature over 150 ºC, tempered glass will not break. Thereby, environment condition generally does not cause tempered glass to break unless temperature exceeding 290 ºC. When there is an increase and decrease of temperature, it causes tension in tempered glass to decline to the point that it is no longer a tempered glass, and consequently the glass will eventually break.

Question: Can heating glass too quickly lead tempered glass to lose its standard? (Lacking standard can be a cause of glass breakage.)

Answer: The key of producing tempered glass is to let glass having tension according to the standard. Tension could occur by heating glass at the temperature of 600-650 ºC, and then cooling down rapidly with quenching process while coming out from the furnace. All of the procedures are controlled by automatic system that has been slightly modified by a controller. The duration for tempering and quenching glass are relatively similar and stable. If glass leaves the furnace too quickly, the temperature of the glass will not reach 600 ºC. This will lead the glass to break in the quenching process as the temperature at 400-600 ºC is the critical temperature of the glass itself. If the glass comes out from the furnace too slowly, the glass will contain numerous roller waves and dots in spread areas. When looking through the glass, its transparency of glass will then be similar to clear stickers rather than general glass. Tension inside tempered glass is an indicator for measuring whether tempered glass has reached its standard or not. If tension is lower than the standard, the breakage of glass will reveal that pieces of that particular glass are larger than usual.

Question: Why does tempered glass break?

Answer: In spite of the fact that tempered glass is 5 times more durable towards smashing forces compared to ordinary glass, tempered glass can be broken down if there are adequate forces. Tempered glass has a weakness in the area of the glass’ corners; therefore, the glass could break if there is only little forces or pressures towards the corners.

Question: How can tempered glass break without any traces of being crashed or despite being in the areas without people passing by? Can tempered glass break even there is no one there?

Answer: Spontaneous glass breakage can occur if there is nickel sulfide (NiS) existing inside the glass. Please see further explanation in the following question.

Question: Why is the severe disperse occur when tempered glass break?

Answer: The process of heating glass to the point that it is soft at the temperature around 600-650 ºC, and then quenching it rapidly by using high-pressure air blast which will harden the glass surface immediately. In this stage, molecules of the glass surface will be arranged disorderly while the interior is still soft. Subsequently, the glass will be cooled down slowly with softer wind until the glass’ temperature has lowered to 40 ºC, and the process will be completed. By being cooled down gradually, glass molecules of the interior have sufficient time to rearrange themselves similarly to crystallization. This dissimilar arrangement causes tension of the surface and the interior to press each other. Tension from the surface pressing each other like wrapping glass altogether is called ‘compressive stress’, and forces pushing out from the interior is called ‘tensile stress’. If tension occurs more than 10,000 psi, it will be called tempered glass; howbeit, there is a regulation stating that it is necessary for tempered glass to have tension more than 14,000 psi. This is because tension will develop tempered glass to be strong and durable towards crashing 5 times more than ordinary glass. Meanwhile, if there are forces that could destroy compressive stress, the glass will not be able to resist tensile stress, causing it to break into small pieces while dispersing severely due to high pressure.

Question: As Tempered glass break because forces destroy compressive stress, then where does forces that could destroy compressive stress come from since there is nothing crash the glass?

Answer: Forces that could destroy compressive stress can be derived from both outer and interior part. Using forces from the outer to destroy compressive stress requires 5 times stronger than the case of ordinary glass destruction, however the corners on the glass serve as a weakness. Using a hammer to softly knock corners of tempered glass can cause it to break down without much forces; hence, installing tempered glass requires a thorough consideration about corners of glass. Fitting areas are necessary to have rubber supports in order to prevent direct interaction to glass and steels. It is important to be aware when installing fitting (e.g. tightening fitting to prevent glass from shaking), otherwise there will be an occurrence in glass distortion. Besides outer forces, internal forces can also destroy compressive stress (spontaneous breaking). It is because of nickel sulfide which is not a component for producing float glass, yet it does not harm float glass in term of clarity. It neither be seen with naked eyes nor affect other properties of the glass. This particular component can be included the production of float glass in various ways such as being contaminated in colors used for making glass or coming from nickel used for covering thermocouple which is a sensor to measure temperature inside the furnace. Nickel can fall over and react with sulfur in the process, resulting in nickel sulfide. This component has two forms including alpha and beta. Alpha form is a stable form, meaning that its shape is permanent. In float glass production, it is necessary to let glass cooling down gradually in order to lessen tension in glass to the lowest level, which generally is less than 200 psi. Thus, furnace used for producing float glass is long. By having a long furnace, NiS that undergoes heat reduction will have time to transfer its form from beta to alpha which has no effect to float glass. Howbeit, during tempered glass production, NiS will be changed back to beta form when heated. As tempered glass will be quenched rapidly, NiS has no time to transform its form back from beta to alpha. Beta molecules ⎯ unstable molecules form of NiS, can get larger overtime, and consequently push other molecules. Tension in tempered glass will constantly increase based on the size of NiS molecules. When compressive stress can no longer tolerate tension of the interior, tempered glass will eventually break.

Question: Is it possible to identify whether tempered glass breaks because of internal or external forces?

Answer: It is impossible to identify pieces of tempered glass that has already been fallen down, however, if the glass is not yet collapsed, we can recognize by examine the center of breakage spot. If there are two big pieces attached with similar size at the center of breakage spot which look alike butterfly wings, it can be identified with almost 100 percent confident that it is because of internal forces. Without those two pieces, the cause of glass breakage is impossible to derive from internal forces.

Question: As spontaneous breakage can occur in tempered glass, why should we call it safety glass?

Answer: The purpose of tempered glass is to be applied in areas that is frequently crashed by people or objects; thus, highly durable glass is required. Likewise, if there is tempered glass breakage, dangers toward people will be much less when compared to ordinary glass that could break into bigger pieces and cause great dangers which can be severe to death. Tempered glass is therefore considered as safety glass.

Question: Can I only select float glass without NiS as a raw material for producing tempered glass?

Answer: No, you can’t. Float glass producers do not have an intention to include NiS into the production, and more importantly, it is impossible to recognize that which lot of float glass contained NiS since it cannot be seen by naked eyes. The discovery of NiS is a study from pieces of broken glass by using electron microscope. In practice, there is still no suitable equipment to find NiS during or after the production.

Question: How often is the breakage caused by NiS?

Answer: International research which gathered information from around the world has discovered that an average of tempered glass breaking spontaneously is 8 in 1,000 sheets.

Question: Do you have any solutions if I do not want tempered glass to be broken by NiS?

Answer: There is one procedure called heat soak process, which is a stimulation process to let NiS enlarge rapidly until tempered glass breaks. The process begins with heating tempered glass until its temperature reaches 290 ºC, and remaining at that particular temperature for 2 hours. According to the research, 95% of tempered glass that contained NiS would break during heat soak process. This means that an average of spontaneous glass breakage will be approximately 4 from 10,000 sheets. It is important to be noted that there is still no procedure to produce NiS-free float glass.

Question: Why is not the heat soaking process included into the production in order to produce the glass with least breakage average?

Answer: Heat soaking is the process that requires high energy to eliminate glass that may break which is 8 from 1,000 sheets of glass. This causes higher production costs, and result in higher prices. By considering the worthiness of this process, if it is not that necessary, we recommend not to use this process. There are various tempered glass from numerous projects that have not been manufactured through heat soak process, yet there are still have no breakage issue. Howbeit, in some projects where spontaneous tempered glass breakage were occurred, other tempered glass from the same lot tended to be broken as well since their raw materials for producing float glass are the same. This tends to cause misunderstanding among users, installers, and producers if there is no clear explanation for what could happen in advance. Therefore, heat soaking process is essential in many projects, e.g., some areas that are difficult to change glass as it requires a large crane to lift glass up or requires high safety standard (such as airports). There are projects that heat soaking process is suggested, e.g., awning roof or ladders that used tempered glass as a raw material. For general projects where glass can be easily changed, heat soaking process is unnecessary as the cost spent on changing glass is less than using heat soaking service for the whole project.

To summarize, an understanding about the breakage of tempered glass among users, installers and producers is essential. Users and installers should be comprehended about risk in using tempered glass. A decision, whether to lessen the risk to the lowest level by heat soaking or bare the risk of spontaneous glass breakage at 8 from 1,000 sheets, is the thing that necessary to be made through a consultation among users, installers, architects, engineers and glass producers. It is noteworthy that there may be no glass broken due to NiS or there are several glass broken from the same lot caused from NiS. More importantly, it is possible that an average of spontaneous glass breaking will be more than 8 from 1,000 sheets as this ratio is an average of total tempered glass produced around the world.

Explained by Atiphong Tunprasert

Intertech Specialty Glass Co., Ltd.